Logic & Quantum

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Logic & Quantum


Field-effect transistors based on band-to-band tunneling (BTBT) have gained a lot of recent interest due to their potential for reducing power dissipation in integrated circuits. In this paper we present a detailed performance comparison between conventional n-i-n MOSFET transistors, and BTBT transistors based on the p-i-n geometry (p-i-n TFET), using semiconducting carbon nanotubes as the model channel material. Quantum transport simulations are performed using the nonequilibrium Green's function formalism including realistic phonon scattering. Under state conditions the drive current and the intrinsic device delay of the TFET are mainly governed by the tunneling barrier properties. On the other hand, the switching energy for the TFET is observed to be fundamentally smaller than that for the MOSFET, reducing the dynamic power dissipation. Aforementioned reasons make the p-i-n geometry well suited for low power applications.

Logic & Quantum


With the continual miniaturization of the MOSFET transistors, power dissipation in integrated circuits has become a major roadblock to performance scaling (ITRS, Website). For more than 30 years, numerous breakthroughs in device and material design have sustained an exponential increase in system performance. The recent introduction of high-k gate oxides into semiconductor technology has also allowed much needed reduction in gate leakage and improved the scalability of future devices. Nevertheless, the physical operational principles of conventional MOSFETs, based on the thermionic emission of carriers over a channel barrier, have imposed fundamental limits on voltage scaling and the reduction of energy dissipation (Lundstrom, 2003: 210).

The subthreshold swing (S) of a conventional MOSFET, which determines the ability to turn off the transistor with the gate voltage (VGS), has a fundamental limit of 2.3BkTq* where kB, T, and q are the Boltzmann constant, temperature, and the electron charge, respectively (S = 60mV/decade at room temperature). Therefore, the requirement of achieving a large on-state current (ION), while maintaining a small off-state leakage (IOFF), has hindered the scaling of the power supply voltage (VDD) in recent years . Consequently, a device with S below the aforementioned conventional limit is desirable for continued voltage scaling, and thereby reducing power dissipation in circuits.

Field-effect transistors based on the band-to-band tunneling (BTBT) phenomenon are being actively investigated due to their potential for low standby leakage (Banerjee & Richardson, 1987: 347). It has been predicted through detailed device simulations that BTBT FETs could produce subthreshold swings below the thermal limit in conventional semiconductor materials such as silicon (Baba, 1992: 455), as well as in carbon nanotube (CNT) based transistors (Koga & Toriumi, 1997: 21). Indeed, this has been experimentally demonstrated in CNTs and more recently with silicon based BTBT FET. BTBT occurs in two different transistor geometries; a popular p-i-n geometry reported in (Hansch & Schulze, 2000: 387) (hereafter called the TFET), and the conventional MOSFET geometry used. In the case of CNT-MOSFETs (Aydin & Zaslavsky, 2004: 1780) it has been established that BTBT is dominated by phonon assisted inelastic tunneling that severely deteriorates the device characteristics. On the other hand, phonon scattering has a less ...
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